Automobile parts and various industrial machine parts each having a shaft shape such as a bolt, a torsion bar, and a stabilizer have been manufactured from a wire rod. Then, in recent years, automobiles and various industrial machines have required a high-strength machine part having a tensile strength of 1200 MPa or more with the aim of reduction in weight and reduction in size.
However, with the achievement of high strength of a machine part, what is called a hydrogen embrittlement, in which due to the effect of hydrogen penetrated into a steel material, a machine part is fractured by stress smaller than that to be expected originally, has become noticeable. The hydrogen embrittlement appears in various forms. For example, in a bolt used for an automobile, a building, and so on, a phenomenon in which after a while since the bolt is fastened, fracture occurs suddenly, called delayed fracture, sometimes occurs.
Then, various examinations for improving hydrogen embrittlement resistance of a high-strength part have been conducted. With regard to a bolt being one example of the high-strength machine part, there has been known a technique utilizing pearlite after wire drawing, as one of techniques improving delayed fracture resistance (Patent Literatures 1 to 4).
However, even by these conventional techniques, it is difficult to improve the hydrogen embrittlement resistance in the high-strength machine part having a tensile strength of 1200 MPa or more. Further, a steel wire and a wire rod suitable for such a machine part are not also invented.